Phosphate esters have heretofore been proposed for use in water as scale inhibitors. For example, Mickus et al U.S. Pat. No. 3,580,855 teaches addition of certain polyphosphoric acid reaction products with polyols at the rate of 5 to 1000 parts per million to water for creation of anti-corrosive, anti-scaling, anti-settling water systems. Apparently, others have also reported scale inhibition in water with similar reaction products.
In Mickus et al, reaction products of polyphosphoric acid with glycerol have previously been produced by heating the reactants together at about 70.degree. to 110.degree. C. for about 4 to 6 hours. Aqueous liquid phase conditions are employed, and water is allowed to accumulate as a byproduct during reaction. This prior art reaction characteristically goes only to about 10 to 25 percent completion (based on inorganic phosphoric acid starting materials). It is generally undesirable to raise the temperature of this reaction in hopes of shifting reaction equilibrium further to the right (towards greater conversion) because the polyol used can tend to degrade or decompose without forming desired products. Thus, glycerol dehydrates fairly easily to acrolein beginning at about 145.degree. C., and higher molecular weight glycols are characteristically even more temperature sensitive as respects thermal degradation.